The disclosure of Japanese Patent Application No. 11-317338 filed on Nov. 8, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an art for raising the temperature of exhaust gas discharged from an internal combustion engine installed in a motor vehicle or the like and, more particularly, to an art for raising the exhaust gas temperature in order to lessen unburned fuel components contained in exhaust gas.
2. Description of the Related Art
With regard to late-model internal combustion engines installed in motor vehicles, it is required that harmful gas components contained in exhaust gas be removed or lessened before the exhaust gas is let out into the atmosphere. To meet this requirement, an art exists in which an emission control catalyst is disposed in an exhaust passage of an engine, and removes or lessens harmful gas components contained in exhaust gas.
Various emission control catalysts have been developed, for example, three-way catalysts, absorption-reduction type NOx catalysts, selective reduction type NOx catalysts, oxidative catalysts, and emission control catalysts combining suitable ones of the aforementioned, etc.
However, all the aforementioned emission control catalysts normally become activated and able to remove harmful gas components in exhaust gas at or above predetermined temperatures. Therefore, when the temperature of an emission control catalyst is lower than such a predetermined temperature, for example, at the time of a cold start of the engine, the emission control catalyst is unable to sufficiently lessen harmful gas components in exhaust gas.
Particularly, when the engine is cold-started, the combustion of an air-fuel mixture tends to be unstable due to the low temperature in the cylinders, so that a relatively large amount of unburned fuel components is discharged from the cylinders. If in such a case, the emission control catalyst is not activated, a relatively large amount of unburned fuel components is emitted into the atmosphere without being subjected to emission control processes.
Therefore, at the time of a cold start of an internal combustion engine, it is critical to quickly activate the emission control catalyst while curbing the amount of unburned fuel components emitted into the atmosphere. To meet this requirement according to the conventional art, a secondary air supply apparatus for an internal combustion engine as described in Japanese Patent Application Laid-Open No. HEI 8-74568 has been proposed.
When the emission control catalyst is in not activated, for example, at the time of a cold start of the engine, the secondary air supply apparatus supplies secondary air into an exhaust passage of the engine to achieve a fuel-lean exhaust air-fuel ratio, so that carbon monoxide (CO) and hydrocarbons (HCs) in exhaust gas are oxidized and substantially eliminated, and so that heat generated by the oxidizing reactions of carbon monoxide (CO) and hydrocarbons (HC) is used to achieve quick activation of the emission control catalyst.
However, since the atmosphere temperature in the exhaust passage is low at the time of a cold start of the engine, it is difficult to sufficiently oxidize and remove carbon monoxide (CO) and hydrocarbons (HCs) merely by achieving a lean exhaust air-fuel ratio through the addition of secondary air to exhaust gas. If the oxidizing reactions of carbon monoxide (CO) and hydrocarbons (HCs) are not sufficiently conducted, the amount of heat generated during the oxidizing reactions becomes small, so that achievement of early activation of the emission control catalyst becomes difficult.
Accordingly, it is an object of the invention to efficiently reduce the deterioration in exhaust emission quality at the time of a cold engine start in an exhaust temperature raising apparatus that raises the exhaust gas temperature while removing the unburned fuel components contained in exhaust gas by providing a technology capable of efficiently achieving the removal of unburned fuel components and raising the exhaust gas temperature.
An exhaust temperature raising apparatus for an internal combustion engine in accordance with an aspect of the invention includes an exhaust passage connected to the internal combustion engine, an exhaust throttle valve that is provided in the exhaust passage and that adjusts an amount of flow of an exhaust gas flowing in the exhaust passage, valve control means for controlling the exhaust throttle valve to a substantially completely closed state at a time when an amount of an unburned fuel component contained in the exhaust gas is to be reduced, engine air-fuel ratio control means for operating the internal combustion engine at a theoretical air-fuel ratio or a fuel-excess air-fuel ratio when the exhaust throttle valve is controlled to the substantially completely closed state by the valve control means, and secondary air supply means for supplying a secondary air to an upstream portion of the exhaust passage when the exhaust throttle valve is controlled to the substantially completely closed state by the valve control means.
In accordance with the internal combustion engine exhaust temperature raising apparatus constructed as described above, when it becomes necessary to reduce the amount of unburned fuel components contained in exhaust gas, the apparatus controls the exhaust throttle valve to the substantially completely closed state, and operates the internal combustion engine at the theoretical air-fuel ratio or a fuel-excess air-fuel ratio, and supplies secondary air into an upstream portion of the exhaust passage.
In this case, since the exhaust throttle valve is set to the substantially completely closed state, the pressure in the exhaust passage from the engine to the exhaust throttle valve is raised, and the flow rate of exhaust gas in the exhaust passage is reduced.
As the pressure in the exhaust passage rises, the atmospheric temperature in the exhaust passage correspondingly rises, so that the temperature decrease of exhaust gas discharged from the engine is reduced.
As a result, high-temperature exhaust gas resides immediately downstream of the engine for a long time. When under such a condition, secondary air is supplied to the exhaust gas, the unburned fuel components in the exhaust gas and oxygen in the secondary air are exposed to high temperatures for a long time, so that the reactions of unburned fuel components with oxygen are accelerated. As the reactions of unburned fuel components with oxygen become active, the amount of heat produced by the reactions increases, so that the temperature of exhaust gas further rises.
The exhaust pressure raised by controlling the exhaust throttle valve to the substantially completely closed state acts on the engine as a back pressure. In that case, however, since the engine is operated at the theoretical air-fuel ratio or a fuel-excess air-fuel ratio, the operation state of the engine does not become unstable.
In the above-described aspect, the time when the amount of unburned fuel components in exhaust gas is to be reduced is, for example, when a relatively large amount of unburned fuel components is discharged from the engine, such as a time when the engine is in a warm-up operation state following a cold start.
Furthermore, in the above-described aspect, if the internal combustion engine is a lean-burn direct-injection internal combustion engine that has fuel injection means for injecting a fuel directly into a cylinder and that is capable of switching between a stratified charge combustion operation and a homogeneous combustion operation, the engine air-fuel ratio control means may cause the homogeneous combustion operation of the internal combustion engine and may cause the fuel injection means to subsidiarily inject the fuel in addition to injection of a main amount of the fuel when the exhaust throttle valve is controlled to the substantially completely closed state by the valve control means so as to reduce the amount of unburned fuel components.
The reason why the homogeneous combustion operation of the engine is performed when the exhaust throttle valve is controlled to the substantially completely closed state is as follows. When the exhaust throttle valve is controlled to the substantially completely closed state, the back pressure acting on the engine rises. If the engine in the stratified charge combustion operation state in that case, the combustion state of the engine may possibly become unstable.
When the fuel injection valve subsidiarily injects fuel (subsidiary fuel), the subsidiary fuel and unburned fuel components, that is, leftover of the main fuel, are burned at high temperatures for a long time, so that the amount of unburned fuel components in exhaust gas is reliably reduced.